Unravelling sediment sources and sinks in the large Ganga river system

Abstract

A basin-scale sediment budget can reveal distinct contribution of tectonic, geomorphological and climatic domains to sediment dispersal in large river systems. While traditional budgets at the reach- and sub-basin-scale have focused on hinterland sediment supply, the role of alluvial storage in modulating sediment fluxes remains poorly understood, particularly in data-scarce Himalayan rivers. To address this, we compiled sediment load data from existing studies across different sub-basins of the large Ganga River system to construct a basin-scale sediment budget. With this integrated sediment budget, we examined spatial variability in sediment generation, transport and deposition across the Himalayan hinterland and alluvial plains. Our analysis reveals strong regional contrasts in sediment dispersal, driven by differences in uplift rates, precipitation and subsidence. The western Ganga rivers, covering 27% of the basin area, contribute only 9% of the total sediment load (385 Mt/a) due to lower uplift rates and precipitation. This results in a supply-limited system with entrenched rivers and minimal sediment storage in the adjacent low-subsiding plains. In contrast, the Eastern Himalayan Hinterland is the dominant sediment source, generating 51% of the total load from 11% of the basin area; however ~45% of this sediment is trapped in the subsiding alluvial plains, forming aggradational channels. The Transitional Himalayan Hinterland bridges these extremes, contributing 23% of the sediment load from the hinterland covering 9% of basin area due to intermediate uplift and precipitation rate. This increases to 29% in the alluvial plains, where low subsidence—similar to the Western Ganga Plains—facilitates efficient sediment transfer downstream. We propose that transient storage in the Eastern Ganga Plains may become a key control on sediment dispersal over a centennial timescale, as increasing extreme precipitation events could shift the system from transport-limited to supply-limited. Accounting for these shifts is essential for effective sediment management and planning.